1. Field of the Invention
This invention relates to processes for the preparation of either naphthenic or paraffinic lubricating oils from a single crude oil feedstock. More particularly, this invention relates to processes wherein a vacuum distillate from a heavy naphthenic crude oil feedstock undergoes controlled hydrocracking into either a high quality naphthenic or paraffinic lubricating oil characterized by exhibiting a low pour point.
2. Description of the Prior Art
Generally two types of lubricating oil basestocks are commonly used for manufacturers of lubricating oils: (a) paraffinic type and (b) naphthenic type. A finished paraffinic basestock has a viscosity index of at least 80 and usually contains (by mass spectral analysis) 5% or more paraffins. Naphthenic basestocks, on the other hand, have viscosity indices less than 80 and contain less than 5% paraffin molecules. The paraffinic oils are used for the manufacture of high quality products such as motor oils, aviation oils and turbine oils which require high viscosity indices. The naphthenic oils are used in less critical applications in which viscosity index is not important. As a reflection of their low paraffin content, naphthenic oils contain little or no wax (&lt;0.1 wt. %) and their pour points are much lower than for a non-dewaxed paraffinic oil of the same molecular weight. To be useful, paraffinic oils must usually be dewaxed to allow fluidity of the oil even at room temperature. It is obvious that the paraffinic and naphthenic oils come from different types of crude oil and that the necessity of dewaxing paraffinic oils increases their cost relative to naphthenic oils.
Lubricating oils are generally prepared by a series of processes which may involve a solvent extraction process for removal of aromatic, asphaltic and sulfur compounds from the lubricating oil cut, solvent dewaxing with propane or a ketone such as methyl ethyl ketone to remove wax, thereby improving the pour and cloud points, clay contacting which is an absorptive process for improvement of color, acid treatment to remove the aromatic unsaturated portions of the distillate or finally, hydrofining to reduce neutralization number and sulfur and to improve color and stability.
The viscosity index and the pour point are two of the most important characteristics of a lubricant. Successful engine lubrication depends upon maintaining an oil film of sufficient viscosity to prevent metal-to-metal contact of moving surfaces. In general, lubricating oils become less viscous with increasing temperature. It is, therefore, important to know how different oils thin out with increasing temperature. The most commonly used means of expressing this viscosity-temperature relationship is called viscosity index. The viscosity index system is based on two standard oils. A highly naphthenic oil from a Gulf Coast Crude which underwent a very considerable decrease in viscosity with increase in temperature was assigned a viscosity index of zero; whereas, a highly paraffinic oil from Pennsylvania which underwent a relatively small decrease in viscosity with increase in temperature was assigned a viscosity index of 100. A method for calculating viscosity index is given in ASTM Method D-567-53.
The viscosity index of an oil is principally dependent upon its chemical composition. Generally speaking, the chemical nature of oils having a high viscosity index makes them more stable in gasoline engines. Thus, oils of high viscosity index are desirable because of their good viscosity-temperature characteristics and because of their chemical stability.
Pour point is defined as that temperature 5.degree. F. above the temperature at which the oil is solid (ASTM Method D-97-57). It is one of the most important characteristics of the lubricant since it represents the limit below which oil cannot flow to the engine parts. If the oil is below its pour point temperature when the engine is started, the oil may not be circulated by the oil pump and the engine may fail because of lack of lubrication. A good quality motor oil should, therefore, always have a pour point at least as low as the lowest temperature at which it might be expected to operate.
The use of solvent refining techniques or polymeric addition agents known as viscosity index improvers has provided means for the regulation and improvement of the viscosity index of lubricating oils. Similarly, the pour point of paraffinic oils can be reduced by solvent dewaxing or the use of additives known as pour depressants.
It would be highly desirable to provide processes for the preparation of high quality paraffinic lubricating oils exhibiting suitably high viscosity indices and low pour points from a single crude oil feedstock. It would be especially desirable to be able to employ a single feedstock and, by regulation of process conditions, be able to obtain either a paraffinic or a naphthenic lubricating oil, as desired. Still further, it would be desirable to provide a versatile lubricating oil manufacturing process which eliminates the need for costly and complex solvent extracting and dewaxing procedures. It would also be desirable to provide processes for the manufacture of high quality lubricating oils from low grade high sulfur heavy naphthenic crude oil feedstocks without the need for preliminary hydrofining to reduce sulfur content to permit conventional processing to be conducted.